For accelerator magnet applications such as the Superconducting Super Collider (SSC) fine filament NbTi superconducting wire is required to minimize distortion of the magnetic fields associated with particle injection. The magnetization of the filaments gives rise to these distortions. The magnetization is proportional to the critical current density (Jc), the volume fraction of NbTi and the filament diameter. The volume fraction of NbTi is usually fixed because of magnet operating current and stability requirements. To minimize the fraction of expensive NbTi, the Jc has to be as high as possible. Therefore, to minimize magnetization the filament diameter has to be as small as possible. Preliminary indications are that filaments 2 microns in diameter would require no correction field windings in the dipole magnets, resulting in a significant reduction in the cost of a giant accelerator.
A significant problem in the production of large quantities of this type of conductor are that so many 2 micron diameter filaments are required to obtain the necessary volume fraction of NbTi. Between 40,000 and 70,000 filaments would be required in such conductors. The manufacture of these conductors normally requires expensive multiple extrusion steps.
As smaller and smaller filaments are produced, they tend to deform non-uniformly leading to filament necking or sausaging. This results in a rapid degradation of obtainable Jc. As interfilament spacing is decreased or as the interfilament material strength is increased, the filament sausaging problem is resolved. However, if the filaments become too close, electrical coupling will result, necessitating the use of costly correction coils. Alternate interfilament matrix materials such as copper-nickel and manganese-doped barriers have to be developed to a point when they can be incorporated practically into an ultrafine filament conductor design. This matrix material change has to be implemented with minimum compromise to the electrical stability of the conductor, presently being provided by the high purity copper matrix.
An interest in ultrafine filament conductors has existed for as long as certain applications of superconductivity, such as A.C. devices and accelerator magnets, have been under consideration. The production of such conductors has been undertaken in response to these needs with varying degrees of success. In 1986, P. Dubots et al., "NbTi Ultra Fine Filament Wires for 50/60 Hertz Use," Adv. in Cryo. Eng. Mtls., Ed. R.P. Reed and A.F. Clark, Vol. 32 Plenum Press, N.Y., 1986, pp. 747, reported on their filaments no less than 1 micron in diameter. However, the Jc of the wire was not anywhere near the values being obtained for well optimized, larger filament conductors. In 1986, for SSC conductor research and development, Intermagnetics General Corp. (IGC) produced SSC inner grade conductor with 2.7 micron filaments (See: C.G. King et al., "Fabrication and Characterization of Fine Filaments of NbTi in Copper Matrix", Adv. in Cryo. Eng. Mtls., Ed. R.P. Reed and A.F. Clark, Vol. 32 Plenum Press, N.Y., 1986 pp. 731, and "Prototype Fabrication of UltraFine Filament NbTi Conductors for SSC", IEEE Trans. on Magnetics, Vol. MAG-23, No. 2, pp. 1351, March 1987.) This conductor had a Jc of 2400 A/mm.sup.2 at 5T, but filament quality was poor, the wire was brittle and the filaments were magnetically coupled. Poor filament quality was determined to be a result of the relatively small number of filaments used in the first extrusion step of the multistep extrusion process used to produce the conductor, that is, the multifilament restack element. The brittle wire was also a result of the conductor geometry, as well as the initial processing variables for the billet, such as, the cold worked state of the alloy, barrier integrity and its bond quality. The coupled filaments were due to the small interfilament spacing in the final size conductor and the lack of an appropriate interfilament matrix material.
Rather than complicate the conductor design with the incorporation of resistive interfilament matrix materials for decoupling of the filaments, it was instead decided to produce 5 micron filament diameter conductor via a single stack multifilament extrusion process and concentrate on cost reduction by obtaining higher current density wire. Significant progress has been made in Jc's of today's production SSC conductors exceeding 3000 A/mm.sup.2 at 5T as compared to 2200 A/mm.sup.2 at 5T in the best Fermi Tevatron conductors.
In another approach and following the suggestion of E.W. Collings, "Stabilizer Design Considerations in Fine Filament Cu/NbTi Composites", Adv. in Cryo. Eng., Eds. A.F. Clark and R.P. Reed, Plenum Press, New York, Vol. 34, pp. 867-878, 1988, E. Gregory et al., (See: "A Conductor With Uncoupled 2.5 Micron Diameter Filaments, Designed for the Outer Cable of SSC Dipole Magnets", IEEE Trans. on Magnetics, Vol. 25, No. 2, pp. 1926, 1989), has doped the interfilament copper matrix with 0.5 wt% manganese. Due to electron-spin-flip scattering by manganese, the filament coupling was shown to be eliminated for filament sizes down to 1.5 microns corresponding to a filament spacing of approximately 0.3 microns. Here large numbers (22,900 & 40,000) of very fine hex shaped monofilament were used which is impractical for large scale production of ultrafine filament conductors. Gregory et al.,s method produced a 5T Jc of 2156 A/mm.sup.2. This value is far from the 2750-3000 range necessary for SSC type superconductors. The "n" value was reported at 13 suggesting poor filament uniformity, which is unacceptable for the intended application.
Since the use of fine filament conductors in some specialized areas of the SSC, in future accelerator applications and A.C. motors and generators could result in a significant cost savings, there is still an opportunity to develop cost effective production techniques aimed at high Jc and n values.
While the foregoing discussion specifically related to multifilament rods of NbTi, it should be appreciated that there is a general need for a multifilament billet having large numbers of rods for various superconductor applications. For example, there is a need for a multifilament billet of Nb.sub.3 Sn type conductors, and others like it.